1. Field of the Invention
This invention relates to a method for carrying out a plurality of liquid-solid contact steps on a single vacuum belt filter. The invention is particularly directed to the processing of a finely divided solid material wherein the processing includes at least one step such as ion-exchange, extraction, leaching, adsorption or the like, in which the rate constant is an important factor and the desired mode of carrying out the reaction is by percolation and the processing also includes at least one other step, such as washing, in which liquid-solid contact time is less than that in the percolation operation.
The invention is especially concerned with the production of fluid zeolitic cracking catalysts by a series of steps including ion-exchange of zeolitic microspheres under percolation conditions allowing for relatively long liquid-solid retention times to satisfy the mass transfer rate content and subsequent rapid washing of the ion-exchanged microspheres with relatively large volumes of another liquid.
Horizontal vacuum belt filters are widely used in the chemical and mineral processing industries. The belt filters comprise an endless permeable belt, usually perforated rubber, supporting a filter cloth and traveling horizontally across a vacuum box. Several models of continuous horizontal vacuum filters are sectionalized to provide separated compartments above the belt. In some units the vacuum box under the belt is subdivided so that the vacuum in the sections above the belt can be individually controlled or the filtrates from the sections can be separately collected. The sectionalized belt filters are unique in that they permit several processing functions to be carried out on a continuous basis on the same piece of equipment. For example, a finely divided slurry can be dewatered, washed and then rewashed on the same filter. The opportunity for conducting a multiplicity of processing steps on a continuous basis on the same unit represents an improvement over processing on a rotary vacuum filter in which the geometry of the unit precludes such processing. However, the sectionalized horizontal vacuum belt filters as heretofore operated are strictly limited in the variety of function which can be conducted with efficiency on the same belt. Thus, the functions that can be carried out serially on the same belt have been limited by the fact that the rate of carrying out any one function was inter-related to the rates of carrying out the other functions. Such rate was dependent upon factors such as belt speed, cake and filter medium resistance and depth of vacuum. In prior methods for processing on a vacuum belt filter, these were constant for all functions.
2. Prior Art
Continuous horizontal belt filters are known in the art. U.S. Pat. No. 2,880,875 to P. W. Alston, "Filtration Apparatus and Method," is directed to such apparatus and its use in filtration. Continuous sectionalized horizontal belt vacuum filters provided with subdivided vacuum boxes are also known in the art. See Perry's "CHEMICAL ENGINEERS' HANDBOOK," at 19-81 (1963). Commercially available horizontal belt filters include the "Eimcobelt" filter and the Straight Line Filter. The latter is described and illustrated in CHEMICAL ENGINEERING CATALOG, CEC, 55th Edition, page 4260, published by Rheinhold Publishing Corporation (1971). The filter illustrated in this catalog includes a sectionalized drainage belt with a vacuum box running the entire effective length of the unit. The vacuum box is divided into three components, all of the same length, and associated with individual outlets, all leading to a single receiver. Illustrated diagrammatically in CEC is a scheme for operating a horizontal belt filter containing three compartments wherein the first provides a cake-forming zone operated under flooded conditions, the second is a washing zone, also operated under flooded conditions, and the third section is a drying zone, operated without flooding. Such processing could obviously not be used in carrying out serially a rate-limited mass transfer function followed by a step such as rapid washing. A flooded feed to a percolation zone would minimize the effectiveness of the percolation step, as would the passage of flooded feed into a washing zone.